Interpretive Summary: The European corn borer (ECB, Ostrinia nubilalis) is a major pest of corn that can cause an excess of $1 billion a year in damages and control costs. The older larvae tunnel into corn stalks disrupting vascular transport and if the infestation is severe enough, it can cause lodging. Importantly, the wound site also provides an entry point for pathogens. To date, very little is known concerning how maize defends itself against ECB stem boring. Scientists at the USDA-ARS Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Florida have been able to identify defensive compounds in maize that are induced in response to ECB feeding. These defenses include benzoxazinoids and kauralexins, two classes of compounds previously shown to have anti-ECB feeding activity. However, when ECB were fed stem tissue expressing these defensive compounds, these rapidly induced chemicals were not capable of inhibiting the short-term growth of ECB. We are currently in the process of studying how ECB are capable of resisting maize defenses. By understanding how ECB are able to either avoid or manipulate maize defenses, we will be able to develop strategies aimed at preventing future ECB infestations.

Technical Abstract:
Plants damaged by insect herbivory often respond by inducing a suite of defenses that can negatively affect an insect’s growth and fecundity. Ostrinia nubilalis (European corn borer, ECB) is one of the most devastating insect pests of maize and in the current study, we examined the early biochemical changes that occur in maize stems in response to ECB herbivory and how these rapidly induced defenses influence the growth of ECB. We measured the quantities of known maize defense compounds, benzoxazinoids and the kauralexin class of diterpenoid phytoalexins. ECB herbivory resulted in decreased levels of the benzoxazinoid, 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one)-ß-D-glucopyranose (DIMBOA-Glc), and a corresponding increase in 2-(2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one)-ß-D-glucopyranose (HDMBOA-Glc). Total quantities of benzoxazinoids and kauralexins were increased as early as 24 hrs after the initiation of ECB feeding. The plant hormones, jasmonic acid (JA) and ethylene (E), and the transcripts encoding their key biosynthetic enzymes also accumulated in response to ECB herbivory consistent with a role in defense regulation. The combined pharmacological application of JA and the E precursor, 1-aminocyclopropane-1-carboxylic acid to stem internode tissue likewise resulted in changes in benzoxazinoids similar to that observed with ECB damage. Despite the fact that maize actively mounts a defense response to ECB stem feeding, no differences in percent weight gain were observed between ECB larvae that fed upon non-wounded control tissues compared to tissues obtained from plants previously subjected to 24 hrs ECB stem herbivory. These rapid defense responses in maize stems do not appear to negatively impact ECB growth suggesting that ECB have adapted to these induced biochemical changes.